I received a few questions about my tip concerning a quicker way to troubleshoot addins during development.
As mentioned in the comments, OneNote needs to exit before rebuilding and replacing your DLL. Since I normally take a while between debugging sessions, this was never a problem for me, but faster people hit this. Make sure OneNote.exe is not running before rebuilding.
If you are writing managed code addins, dllhost.exe should also get shut down. It should close automatically when OneNote exits, but if you are experiencing problems, check to make sure it is not running either. Other office applications using managed code addins may have an instance of this running as well - keep any eye out for that.
I hope this clears up any confusion.
Questions, comments, concerns and criticisms always welcome,